1. Field of the Invention
The present invention generally relates to non-contact sensors for monitoring surface variations of a component part, and more specifically relates to a non-vibrating capacitance probe which uses a variable capacitor to measure the contact potential difference between two surfaces, generally on the same component part, and thereby recognizes surface variations such as wear of an object subjected to, for example, a sliding contact.
2. Technical Field
Mechanical systems such as heat combustion engines have components that are dynamically in contact with another body. These components are subjected to cyclic motions that can involve impact loading, shear straining, plastic deformation, frictional heating, and fatigue of sub-surface regions. A combination of these mechanisms often leads to surface damage that impairs the performance of the component. In addition, the chemical interaction between the component surface and surrounding fluids also can accelerate surface degradation. Such problems, if unattended, can result in catastrophic malfunction of the machine and even compromise operational safety. In this regard, it is desirable to monitor the surface condition of a critical tribocomponent. The design of sensors to monitor the surface condition of the tribocomponents and the operation of machinery depends largely on the nature of tribological application.
A surface-monitoring method that exploits the spatial variation in the work function of a material is presented herein. The work function refers to an energy barrier to prevent the escape of electrons from the surface of the material. The work function is governed by the physio-chemical nature of the surface and also depends on the environmental conditions. From a tribological standpoint, the work function is a useful parameter for evaluating mechanical deformation features such as dislocation pile-ups and residual stresses. For example, it has been demonstrated that a metal subjected to different degrees of compressive stress exhibits a variation in the work function. Craig, P. P. and Radeka, V., “Stress Dependence of Contact Potential: The ac Kelvin Method,” Rev. Sci. Instrum., Vol. 41, pp. 258-264, 1969. The present invention is a non-vibrating capacitance probe as modified from that of the Kelvin-Zisman method, Zisman, W. A., “A New Method of Measuring Contact Potential Differences in Metals,” Sci. Instrum., Vol. 3, pp. 367-370, 1932, that uses a variable capacitor to measure the contact potential difference (CPD) between two surfaces.